This invention relates to a method of aligning disks having a flat section and a device therefor. More particularly, this invention relates to a method of and a device for aligning semiconductor wafers of the usual shape having indexing flats such that their flats can be uniformly oriented in a specified angular direction inside a cassette.
In manufacturing integrated circuit structures by modern techniques, semiconductor wafers are transported to various processing stations and handled in different manners. Since these structures are extremely contamination-sensitive especially in dense, complex LSI and VLSI circuits, many automated wafer-handling apparatus have been proposed, for example, in U.S. Pat. Nos. 4,311,427 and 4,449,885. In U.S. patent application Ser. No. 693,722 filed Jan. 22, 1985 and assigned to the present assignee, there is disclosed a robotic wafer-handling mechanism whereby wafers are sequentially taken out of a cassette of a conventional form and placed individually on a pedestal for ion-implantation. Since wafers of the conventional shape are generally circular and provided with a flat edge formed along a chord (which has been variously referred to, for example, as "a guide flat" and "an indexing flat"), each pedestal is also formed substantially in the same shape as the wafer so that its surface will be completely shielded from impinging ions by a wafer placed thereon. In order to successfully shield the pedestal surfaces, therefore, each wafer must be correctly oriented with respect to the pedestal onto which it is placed. Since the mechanism for transferring wafers from a cassette to the pedestals is not equipped with a "wrist" to control the orientation of the flat section of each wafer, the wafers must be aligned with their flat sections oriented in a proper direction before they are brought to this robotic wafer-handling apparatus.
Various devices have heretofore been proposed for aligning disks such as semiconductor wafers of the type having an indexing flat along a chord. Mechanisms for aligning wafers singly have been described, for example, in U.S. Pat. Nos. 3,297,134, 3,982,627 and 3,997,065. Aforementioned U.S. Pat. No. 4,311,427 discloses a device for aligning a plurality of wafers simultaneously while they are supported vertically in a conventional cassette. This simple device makes use of a pair of opposed rollers to find the flats and to orient them uniformly in the downward direction. It is not adapted to align the wafers in a specified angular direction. More recently, U.S. Pat. No. 4,662,811 disclosed apparatus capable of orienting the flats of wafers nearly in any position. In order to keep the wafers rotating, however, rollers with a non-circular cross-section and rollers which must be moved vertically up and down were introduced. Apparatus with such rollers are more difficult to design to avoid wafer surface contamination due to the mechanisms required and expensive to manufacture.